1. Field of the Invention
This invention relates to a Fexe2x80x94Ni based permalloy suitable for use in a magnetic head, a magnetic shielding material, an iron core of a transformer or the like and having excellent magnetic properties and a method of producing the same as well as a cast slab.
2. Description of Related Art
As the Fexe2x80x94Ni based high magnetic permeability alloy or so-called permalloy, there are usually typified PB material (40-50 wt % Ni), PC material (70-85 wt % Nixe2x80x94Moxe2x80x94Cu), PD material (35-40 wt %-Nixe2x80x94Fe) and the like, which are defined according to JIS C2531. Among these alloys, the PB material is mainly used in applications utilizing the characteristic that saturated magnetic flux density is large, such as stator in a watch, pole piece in an electromagnetic lens and the like, while the PC material is used as a high sensitivity transformer or a magnetic shielding material at a high frequency zone utilizing an excellent permeability. Among these alloys, it is designed to cope with applications such as a magnetic head, a shield case and the like by adding an additional element such as Nb, Cr or the like to provide the abrasion resistance and corrosion resistance (for example, JP-A-60-2651).
As another example of improving the properties of these alloys, JP-A-62-142749 and the like disclose that the permeability and the punching property are improved by adjusting impurity elements such as S, O and the like. Recently, the movement from PC material to PB material or from PB material to PD material is observed for reducing the cost, or there is adopted a method of supplementing for the lack of material properties by designing a fabricator.
In the material makers, therefore, it is strongly noticed to develop materials such as PB material having properties corresponding to those of PC material or PD material having properties corresponding to those of PB material, This increases a degree of freedom in the design of fabricator and hence is effective to give products having higher performances to markets.
It is, therefore, an object of the invention to provide a Fexe2x80x94Ni based permalloy satisfying the above demand. That is, the invention is to improve the magnetic properties of PB material and PD material to grade up to the magnetic properties corresponding to those of PC material and PB material and to further improve the magnetic properties of PC material and to develop materials capable of coping with applications of high sensitivity and frequency.
The inventors have made various studies in order to achieve the above object and found that Fexe2x80x94Ni based permalloys having the following constructions are preferable and as a result the invention has been accomplished.
The invention lies in a Fexe2x80x94Ni based permalloy comprising Ni: 30-85 wt %, C: not more than 0.015 wt %, Si: not more than 1.0 wt %, Mn: not more than 1.0 wt %, P: not more than 0.01 wt %, S: not more than 0.005 wt %, O: not more than 0.006 wt %, Al: not more than 0.02 wt % and the remainder being Fe and inevitable impurities, provided that Ni segregation amount CNis represented by the following equation is not more than 0.15 wt %, preferably 0.10 wt %.
CNis=analytical value of Ni component (wt %)xc3x97CiNis(c.p.s.)/CiNiave.(c.p.s.) 
wherein CiNis is a standard deviation of X-ray intensity (c.p.s.) and CiNiave. is an average intensity of all X-ray intensities (c.p.s.).
In addition to the above constitutional components, the alloy according to the invention is favorable to contain not more than 15 wt % of at least one selected from the group consisting of Mo, Cu, Co and Nb within a range of not more than 20 wt % in total.
And also, the alloy according to the invention is favorable to control an amount of non-metallic inclusion having a diameter corresponding to a circle of not less than 0.1 xcexcm to not more than 20 particles/mm2, preferably not more than 10 particles/mm2.
Furthermore, the alloy according to the invention is favorable to have the following constructions:
(1) The alloy containing 35-40 wt % of Ni exhibits such magnetic properties that a maximum magnetic permeability xcexcm is not less than 50000, an initial magnetic permeability xcexci is not less than 10000 and a coercive force Hc is not more than 0.05 (Oe);
(2) The alloy containing 40-50 wt % exhibits such magnetic properties that a maximum magnetic permeability xcexcm is not less than 100000, an initial magnetic permeability xcexci is not less than 30000 and a coercive fore Hc is not more than 0.02 (Oe);
(3) The alloy containing 70-85 wt % exhibits such magnetic properties that a maximum magnetic permeability xcexcm is not less than 400000, an initial magnetic permeability xcexci is not less than 20000 and a coercive force Hc is not more than 0.006 (Oe).
And also, the invention proposes a method of producing a Fexe2x80x94Ni based permalloy, which comprises continuously casting an alloy comprising Ni: 30-85 wt %, C: not more than 0.015 wt %, Si: not more than 1.0 wt %, Mn: not more than 1.0 wt %, P: not more than 0.01 wt %, S: not more than 0.005 wt %, O: not more than 0.0060 wt %, Al: not more than 0.02 wt %, and, if necessary, not more than 15 wt % of at least one selected from the group consisting of Mo, Cu, Co and Nb within a range of not more than 20 wt % in total and the remainder being Fe and inevitable impurities into a slab, and subjecting the continuously cast slab to a homogenizing heat treatment and further to a hot rolling.
In the production method according to the invention, it is favorable that the continuous casting is carried out without applying an electromagnetic agitation, and that a cast slab for the permalloy having an area ratio of equiaxed crystal in a cast texture of a continuously cast slab of not more than 1% is used.
As the homogenizing heat treatment, it is favorable that the continuously cast slab is treated at a temperature of 1100-1375xc2x0 C. under a condition that Ni diffusion distance DNi represented by the following equation is not less than 39:
DNi=(D t)xc2xd/xcexcm 
wherein
D: diffusion coefficient, D=D0xc3x97exp (xe2x88x92Q/RT),
D0: vibration number item=1.63xc3x97108/xcexcm2 sxe2x88x921 
Q: activation energy of Ni diffusion=2.79xc3x97105/J mol1 
R: gas constant=8.31/J mol1 Kxe2x88x921 
T: temperature/K
t: annealing time/s
Furthermore, a cold rolling is carried out to produce a product after the hot rolling step, if necessary. And also, it is favorable to conduct a magnetic heat treatment of 1100-1200xc2x0 C. after the cold rolling step. Such a magnetic heat treatment is favorable to be carried out in a hydrogen atmosphere.
Moreover, the cold rolling step may include usually used steps such as annealing, BA, pickling and the like. And also, the cast slab used herein may include a cast ingot for the formation of usual ingot in addition to the continuously cast slab.